Procedure and device for the purification of exhaust gas

ABSTRACT

The invention relates to a procedure for purifying exhaust gas in the exhaust gas duct of a combustion engine with an exhaust gas purification system and a SCR-catalyzer that is arranged in the exhaust gas purification system for a selective catalytic reduction of nitrous gases (SCR), whereby reducing agents for the selective catalytic reduction of nitrous gases can be added to the exhaust gas in direction of the flow before the SCR-catalyzer. According to the invention it is provided that the exhaust gas is led through a catalyzer for converting nitrous gases with hydrocarbons and subsequently through the SCR-catalyzer and that the operating parameters are adjusted in a first operating phase in such a way, that nitrous gases are mostly converted at the catalyzer for converting nitrous gases with hydrocarbons and that the operating parameters are adjusted in a second operating phase in such a way, that nitrous gases are mostly converted at the SCR-catalyzer. The adjustments for the first operating phase are selected at the start of the combustion engine. The catalyzer is heated to its operating temperature in this operating phase for the conversion of nitrous gases with hydrocarbons. Because it is arranged before the SCR-catalyzer and a particle filter that is usually arranged in front of it, its heating takes place very fast and the output of nitrous gases in the starting phase of the combustion engine can be reduced as opposed to the state of the art. The normal operation of the combustion engine is characterized by a lean mixture composition and therefore by an oxygen-rich exhaust gas. This is adjusted in the second operating phase, in which nitrous gases are converted in the SCR-catalyzer that is now heated to its operating temperature.

TECHNICAL FIELD

The invention concerns a procedure for purifying exhaust gas in the exhaust gas duct of an internal combustion engine with an exhaust gas purification system and a SCR-catalyzer that is arranged in the exhaust gas purification system for the selective catalytic reduction of nitrous gases (SCR), whereby reducing agent for the selective catalytic reduction of nitrous gases can be added to the exhaust gas in the direction of the flow before the SCR-catalyzer.

BACKGROUND

Related to statutory provisions regarding the nitrogen oxide emission of motor vehicles an exhaust gas after-treatment is required. The selective catalytic reduction (SCR) can be used for reducing the nitrogen oxide emission (de-nitrification) of combustion engines, in particular of diesel engines, with temporally mainly lean, thus oxygen-rich exhaust gas. A defined amount of a selectively acting reducing agent is hereby added to the exhaust gas. This can for example be ammoniac, which is added directly in the form of gas and which is also won from a precursor substance in the form of urea or from a urea-water solution (HWL). The reducing agent can also be produced from ambient air and fuel in a plasma procedure with a subsequent conversion of educts in a catalyzer and/or a storage catalyzer. At the practical use in self-igniting combustion engines a particle filter is upstream before the SCR-catalyzer. At the start of the combustion engine the particle filter has first to be heated by the hot exhaust gas and delays thereby the achieving of the operating temperature of the SCR-catalyzer. The heating is accelerated by power-operated measures, whereby the consumption of fuel can increase by 1.5% to 2% in a test cycle.

In order to eliminate nitrous gases from the exhaust gas they can be converted with hydrocarbons to carbon dioxide and nitrogen in an alternative procedure at applicable catalytically acting surfaces. For this purpose hydrocarbons are added to the exhaust gas flow by metering fuel or by an applicable controlling of the combustion in the combustion engine. Such an exhaust gas purification system is described in EP 062 1400 B1. In EP 062 1400 B1 it is achieved by selecting a qualified point of time and metering amount of an injection of fuel into the combustion chamber of a combustion engine, that the exhaust gas purification system quickly reaches its operating temperature and that the amount of produced hydrocarbons causes an almost complete conversion of nitrous gases at the exhaust gas purification system without an excessive output of unburned hydrocarbons.

The reaction at a catalytically acting surface for converting nitrogen oxides by hydrocarbons brings already a high conversion rate from 250° C. to 300° C. The conversion rate depends on the oxygen amount in the exhaust gas and reaches a maximum in the range of 0.7% oxygen. But the conversion rate of such an exhaust gas purification system is not sufficient at the oxygen-rich exhaust gas of self-igniting combustion engines in normal operation.

It is the task of the invention to provide a procedure and a device for the exhaust gas purification, which allow a high conversion rate for nitrogen oxides shortly after the start of a combustion engine as well as in normal operation with oxygen-rich exhaust gas.

SUMMARY

The task of the invention is solved thereby that the exhaust gas is led through a catalyzer for converting nitrous gases with hydrocarbons and subsequently through the SCR-catalyzer and that the operating parameters are adjusted in a first operating phase in such a way, that nitrous gases are mostly converted at the catalyzer for converting nitrous gases with hydrocarbons and that the operating parameters are adjusted in a second operating phase in such a way that nitrous gases are mostly converted at the SCR-catalyzer. The adjustments for the first operating phase are selected at the start of the combustion engine. The catalyzer for converting nitrous gases with hydrocarbons is heated to its operating temperature in this operating phase by power-operated measures. Because it is arranged before the SCR-catalyzer and a particle filter that is usually arranged in front of it, its hating takes place very quickly and the output of nitrous gases can be reduced in the starting phase of the combustion engine as compared to the state of the art. If this first catalyzer has reached its operating temperature, it is impinged with hydrocarbons and it converts the nitrous gases that are produced in the combustion engine. The normal operation of the combustion engine is characterized by a lean mixture composition and therefore by an oxygen-rich exhaust gas. This is adjusted in the second operating phase, in which he nitrous gases can be converted in the SCR-catalyzer that is now heated to its operating temperature.

An improved conversion rate at the catalyzer for converting nitrous gases with hydrocarbons is achieved, by adjusting the operating parameters in the first operating phase in such a way that the oxygen amount in the exhaust gas is reduced as opposed to a normal operation of the combustion engine, that the exhaust gas temperature is raised at the catalyzer for converting nitrous gases with hydrocarbons to at least 250° C. and that the concentration of unburned hydrocarbons in the exhaust gas is increased. The reaction in the catalyzer is exothermic, so that it contributes to the heating of the subsequent particle filter and the SCR-catalyzer. The increase of the concentration of unburned hydrocarbons in the exhaust gas can be achieved according to the state of art by adding fuel to the exhaust gas, throttling the supply air of the combustion engine, increasing the exhaust gas recirculation rate or by a late injection or an injection in the output cycle at combustion engines with direct injection.

If the oxygen amount in the exhaust gas is adjusted in the first operating phase to lower or equal 5%, preferably in a range between 0.6% and 0.8%, a very high conversion rate of the catalyzer for converting nitrous gases with hydrocarbons can be achieved.

If the operating parameters are adjusted in the second operating phase in such a way, that the combustion engine is operate din normal operation with a lean fuel-air mixture and that the concentration of unburned hydrocarbons is reduced compared to the first operating phase, it can be taken advantage of the range with low fuel consumption of the combustion engine. In this operating phase nitrous gases are reduced in the SCR-catalyzer, which reduces even under oxygen-rich conditions, as they occur in the exhaust gas at normal operation of a self(igniting combustion engine.

One alternative of the procedure, which considers the conversion features of the components of the exhaust gas purification system very well, provides that the switchover from the first to the second operating phase takes place when the SCR-catalyzer reaches a default operating temperature.

The task of the invention that concerns the device is solved by arranging a catalyzer for converting nitrous gases with hydrocarbons in the exhaust gas duct close to the combustion engine and by arranging the SCR-catalyzer in the direction of the flow of the exhaust gas after the catalyzer for converting nitrous gases with hydrocarbons. It can be achieved by this arrangement that the catalyzer for converting nitrous gases with hydrocarbon quickly reaches its operating temperature in a first operating phase and effectively causes the conversion of nitrous gases already shortly after starting the combustion engine.

A purification of the exhaust gas of the combustion engine from particle filters is achieved by arranging a particle filter in the exhaust gas duct between the catalyzer for converting nitrous gases with hydrocarbon and the SCR-catalyzer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in detail in the following by an embodiment that is illustrated in the figures.

It is shown in:

FIG. 1 is a schemat illustration of the technical environment, in which the invention can be used,

FIG. 2 is a diagram for the conversion rate of a catalyzer for converting nitrous gases with hydrocarbons.

DETAILED DESCRIPTION

FIG. 1 schematically shows an illustration of the technical environment, in which the invention can be applied. A combustion engine 10 is shown, which can be arranged as diesel engine, with a fuel metering 11, a combustion air supply 12, in which combustion air 16 is conducted, and an exhaust gas duct 17, in which the exhaust gas flow of the combustion engine 10 is conducted. A pressure increase step 15, which is driven by an exhaust gas turbine 18, and a throttle valve 14 are arranged along the combustion air supply 12 in the direction of the flow of the combustion air 16. An exhaust gas recirculation 13 connects the combustion air supply 12 with the exhaust gas duct 17. The exhaust gas turbine 18 as well as components of an exhaust gas purification system 20, a lambda probe 21, a first catalyzer 22, a temperature sensor 23, reducing agent add-on 24 and a SCR-catalyzer 25 for the selectively catalytic reduction of nitrous gases are arranged in the direction of the flow of the exhaust gas flow after the combustion engine 10. The exhaust gas is delivered over an exhaust gas output 26. The first catalyzer 22 is arranged as a catalyzer for converting nitrous gases with hydrocarbons and combines with a subsequent particle filter.

The combustion engine 10 is supplied with combustion air 16 over the combustion air supply 12 from outside. The conducted air amount can be adjusted by the throttle valve 14. For reducing the pollutants exhaust gas from the exhaust gas duct 17 can be mixed to the combustion air 16 over the exhaust gas recirculation 13 in amounts that depend on the operating parameters of the combustion engine. The oxygen content in the exhaust gas can also be influenced in particular by the throttle valve 14 and the exhaust gas recirculation 13 in order to adjust advantageous parameters for the conversion rate for nitrous gases of the first catalyzer 22.

Directly after the start of the combustion engine 10 the first catalyzer 22 is heated to an operating temperature of 250° C. to 300° C. Because it is arranged close to the exit of the combustion engine 10, it reaches it comparably fast. The oxygen amount of the exhaust gas is reduced in this operating phase, in order to achieve a sufficient conversion rate of the first catalyzer 22. Furthermore hydrocarbons are produced by power-operated measures, which are required for the conversion of nitrous gases from the combustion engine 10 in the first catalyzer 22. In the following operation the particle filter that is arranged at its exit and the SCR-catalyzer 25 that is arranged after the reducing agent add-on 24 are heated. The course of the heating can be traced with the temperature sensor 23, so that at qualified operating parameters it can be switched over to a conversion of nitrous gases by the SCR-catalyzer 25. The combustion engine 10 can then switch over in a fuel-saving normal operation, at which it is supplied with a lean fuel-air mixture. A reducing agent, as for example a urea-water solution, is added to the SCR-catalyzer 25 over the reducing agent add-on 24 in this operating phase.

FIG. 2 shows a diagram 30 for the conversion rate of the catalyzer for converting nitrous gases with hydrocarbons. Along a temperature axis 36 the conversion rate is put along the conversion rate axis 31 for different oxygen contents in the exhaust gas. A first curve 32 shows the course of the conversion rate for oxygen-free exhaust gas. A second curve 33 shows the course of the conversion rate for exhaust gas with 0.7% oxygen. A third curve 34 and a forth curve 35 show the conversion rates for 2% or 8% oxygen content in the exhaust gas. It can be taken from the diagram 30 that a high conversion rate of the catalyzer for converting nitrous gases with hydrocarbons can be achieved when a temperature over 250° C. is reached and when the oxygen content in the exhaust gas lies at 0.7%. At oxygen contents over 2% the conversion rates shrinks, so that the system for the exhaust gas purification qualifies in the starting phase, when the oxygen content in the exhaust gas is reduced as opposed to a normal operation of a lean operated combustion engine. 

1. A method of purifying an exhaust gas in an exhaust gas duct of a combustion engine with an exhaust gas purification system and a SCR-catalyzer that is arranged in the exhaust gas purification system for a selective catalytic reduction of nitrous gases, wherein at least one reducing agent for the selective catalytic reduction of the nitrous gases is added to the exhaust gas in a direction of flow before the SCR-catalyzer, the method comprising: leading the exhaust gas first through a catalyzer for converting nitrous gases with hydrocarbons and second through the SCR-catalyzer, wherein one or more operating parameters are adjusted in a first operating phase whereby the nitrous gases are more substantially converted at the catalyzer, and wherein the one or more operating parameters are adjusted in a second operating phase whereby the nitrous gases are more substantially converted at the SCR-catalyzer.
 2. A method according to claim 1, further comprising adjusting the one or more operating parameters in the first operating phase to reduce an oxygen amount in the exhaust gas in comparison to a normal operation of the combustion engine, wherein an exhaust gas temperature at the catalyzer for converting nitrous gases with hydrocarbons is raised to at least 250° C. and in that the concentration of unburned hydrocarbons in the exhaust gas is increased.
 3. A method according to claim 2, further comprising reducing the oxygen amount in the exhaust gas in the first operating phase to less than or equal to 5%, preferably between 0.6% and 0.8%.
 4. A method according to claim 1, further comprising adjusting the one or more operating parameters in the second operating phase to operate the combustion engine in a normal operation with a lean fuel-air mixture and to reduce the concentration of unburned hydrocarbons in comparison to the first operating phase.
 5. A method according to claim 1, further comprising switching from the first into the second operating phase when the SCR-catalyzer has reached a default operating temperature.
 6. A device configured for purifying an exhaust gas in an exhaust gas duct of a combustion engine with an exhaust gas purification system and a SCR-catalyzer that is arranged in the exhaust gas purification system for a selective catalytic reduction of nitrous gases, wherein at least one reducing agent for the selective catalytic reduction of the nitrous gases is added to the exhaust gas in a direction of flow before the SCR-catalyzer, comprising: a catalyzer for converting nitrous gases with hydrocarbons, wherein the catalyzer is arranged in the exhaust gas duct in close proximity to the combustion engine, and wherein the SCR-catalyzer is arranged in a direction of flow of the exhaust gas after the catalyzer for converting nitrous gases with hydrocarbons.
 7. The device of claim 6, wherein a particle filter is arranged in the exhaust gas duct between the catalyzer for converting nitrous gases with hydrocarbons and the SCR-catalyzer. 